I was at 18,7 km and it worked. You can also test it with the GPS emulator component. Plug RX and TX wires from FTDI / Prolific cable onto OSD board, power it on normaly (not on the programming mode), start the GPS emulator and set a high speed course in any direction.

You have it wired in the Pic to the ENG voltage side. If you want amperage you have to go to one of the ADC channels. I would recommend wiring G and V to the ENG connections, and the SIG to ADC 6. This will give you ENG voltage and ENG amperage. (There are examples on remzibi's site)